In the polymerization of olefins, it is important to prevent the resulting polymer from breakage, and to this end, a treatment for increasing the strength of catalyst grains themselves has generally been carried out by a suitable method, typical of which is a prepolymerization treatment of the catalyst component.
This treatment comprises polymerizing an olefin in a small amount in the presence of the catalyst component and an organoaluminum compound before subjecting the catalyst component to polymerization of the olefin, and thereby incorporating the resulting polymer into the catalyst component. However, when this prepolymerization treatment is carried out on the so-called catalyst of magnesium-support type, the activity of the catalyst is largely lowered after storage for a long period of time.
In order to suppress the deterioration of the catalyst, a method has been employed comprising washing the catalyst after being subjected to the prepolymerization treatment with a large amount of an organic solvent, but the effect thereof is not sufficient.
Of late, it has been proposed to subject, before supporting a titanium component on a magnesium-containing solid, the magnesium-containing solid to a prepolymerization treatment, thus preventing deterioration of the catalyst (Japanese Patent Laid-Open Publication Nos. 89508 to 89511/1988). However, the catalyst components described in these publications consist of combinations of particular compounds and cannot exhibit high catalytic activity.
The inventors have hitherto proposed a catalyst component for polymerization of olefins, which is obtained by contacting metallic magnesium, halogenated hydrocarbons and a compound represented by the general formula X.sub.n M(OR).sub.m-n to obtain a solid, contacting the resulting solid with a halogen-containing alcohol and then contacting with an electron-donating compound and titanium compound (Japanese Patent Laid-Open Publication No. 264607/1988). This catalyst component has a polymerization capacity sufficient for practical use and results in polymer powders excellent in grain property, but has a problem of deterioration during storage.
The inventors have found that it is effective to bring the above described catalyst component, during preparation thereof, into contact with titanium alkoxide, before contact with a halogen-containing alcohol, and then to contact with an olefin in the presence of an organoaluminum compound. The present invention is based on this finding.